Saturable core transformer



Jan. 20, 1959 F. w. KELLEY, JR

SATURABLE coma: TRANSFORMER 2 Sheets-Sheet 1 Filed Oct. 16, 1956Invenior:

Fred W. Kelley Jr. y (i/ 4% His Ahorney Jan. 20, 1959 F. w KELLEY, JR

SATURABLE coma TRANSFORMER 2 Sheets-Sheet 2 Filed Oct. 16, 1956lnvenlor: Fred W. Kelley Jr.

i AHorney I United States SATURABLE CORE TRANSFORMER Fred W. Kelley,.lr., Melrose, Mass, assignor to General Electric Company, a corporationof New York Application October 16, 1956, Serial No. 616,311 9 Claims.(Cl. 323-56) The present invention relates to improved saturable coreapparatus and, more particularly, to magnetic-core control equipmentwherein the functions of a transformer and amplistat are present in aunitary construction affording high gain.

Saturable core reactance devices of the type including A.-C. outputwindings and D.-C. saturating or control windings wound about the samecore have gained widespread use in supplementing or replacing electronicamplifiers and rotating regulators, especially where the magneticamplifier incorporating such devices also include rectifiers in certaincircuit relationships which yield increased gain. Magnetic amplifiers ofthis construction have become identified by the term amplistat, andcommonly it is required that the amplistat be excited by or in uniquecircuit cooperation with a transformer.

Among the cardinal advantages of magnetic amplifiers are theirrelatively light weight, sturdiness, long life expectancy, wholly staticoperation, and adaptability to good impedance matching in mostapplications. Transformers share many like advantages, and since the twoare frequently utilized in conjunction with one another, theircombination into unitary structures wherein flux paths are commonappears attractive. Prior efforts in this direction have resulted in thedesign of simple saturable reactance elements combined withtransformers, although such combinations do not provide sufficientlyhigh gain for many purposes and the resulting winding and corestructures are complex. In combination units, a further importantconsideration is that control circuit impedances in the saturablereactor circuitry should preferably be of low value, such that theseimpedances need not be of excessive bulk and weight and such thatmatching with low impedance control components may be readily achieved;yet this requirement further militates against high gain and leads toundesired power losses.

Accordingly, it is one of the objects of the present invention toprovide an improved and simplified magnetic device wherein operatingcharacteristics of a transformer and a high gain magnetic amplifier arerealized.

A further object is to provide a transformer and cooperating amplistatassembly of minimum size and Weight and having a low impedance controlcircuit.

Another object is to provide unique core and winding arrangements whichoccasion heightened gain and curtail power losses intransformer-saturable reactance circuitry.

By way of a summary account of this invention in one of its aspects, Iprovide three closed magnetic cores each providing a closed magneticflux path. Single phase A.-C. excitation is applied to a transformerprimary winding about one of these cores, which is not saturated, thetransformer secondary being in two halves each surrounding the same coreand each also surrounding a different one of the other two cores. AD.-C. control signal winding surrounds both of these other two cores andcontrols their saturation. Output is delivered to a load coupled to acenter tap intermediate the two secing effects of rectifiers 11 and 12.

(at ondary halves and to each of the secondary ends through a dry platerectifier, each half of the secondary thereby delivering alternatepulses of unidirectional current to the load. Load current is responsiveto the magnitudes of direct current caused to flow through the controlsignal winding.

Although the features of this invention which are believed to be novelare set forth in the appended claims, further details of the inventionand the objects and advantages thereof may be readily comprehendedthrough reference to the following description taken in connection withthe accompanying drawin wherein:

Figure 1 is a schematic diagram of a single phase transformer-amplistatembodying the present teachings and arranged to provide A.-C. outputs;

Figure 2 illustrates pictorially one core and Winding assemblycorresponding to that employed in the arrangement of Figure 1;

Figures 3 and 4 are schematic portrayals of bi-phase and bridge typecircuits, respectively, which embody this invention in providing D.-C.output signals;

Figures 5 and 6 depict schematically two circuit arrangements forenergizing D.-C. loads responsive to threephase excitations; and

Figure 7 provides a pictorial view, in perspective, of one preferredthree-phase transformer-amplistat core and winding assembly.

The arrangement presented in Figure 1 derives excitation from an AC.source 1 and delivers controlled A.-C. output to a suitable load 2, withboth a transformer action and high-gain saturable reactance controlaction occurring. Control is achieved through the usual type ofadjustment of direct currents in a control signal win-ding '3, as bymanual or automatic variation of the effective values of an impedance 4serially coupled with a direct current source 5 and control winding 3.Three closed flux paths are created by three closed cores: a controlledcore 6; and two auxiliary or control cores 7 and 8. Turns of theaforementioned signal winding 3 encircle both of the control cores 7 and8, while the output or secondary winding 9 surrounds only the controlledcore 6. A primary winding which also functions as a form of reactorwinding appears in two independent halves, 10a and 10b, the turns ofeach encircling both the controlled core 6 and a different one of thecontrol cores 7 and 8. Winding half 10a is coupled across the A.-C.source through a dry contact rectifier 11, which may he a selenium orgermanium rectifier, for example, and the other primary winding half 10bis also coupled across the same source through a serially-connected drycontact rectifier 12 which is of a polarization opposite to that ofrectifier 11.

In the aforesaid electrical and magnetic circuit arrangement, theprimary winding halves Illa and 10b are each excited by differentalternate half cycles of the alternating current supply 1, due to theselective block- As each of these winding halves is excited in thisperiodic manner, it tends to drive magnetic flux through the controlledcore 6 in a direction opposite to that occasioned by the other windinghalf and, thereby, to induce A.-C. output signals in the load outputwinding 9. However, the inductive reactances exhibited by winding halves10a and 10b in conjunction with their cores remain very high unless oneof the cores associated therewith is saturated, and thus negligiblepower is absorbed from the source and delivered to the load withoutoccurrence of certain saturations. These saturations do not appear inthe controlled core 6, because the windings Illa and 10b, core 6, andsource 1 are designed in known manner to preclude such occurrences.Instead, the control cores 7 and 8 are saturated periodically underinfluence of 3 the pulsating fluxes produced therein by windings 10a and10b and superimposed upon the unidirectional fluxes produced therein bythe DC. control winding 3.

Assuming for the moment that the instantaneous values of A;-C. voltagedelivered by source 1 are rising from a zero level in a sense to causecurrent flow through primary winding 10a, and that a certain value ofD.-C. flux insufficient to saturate control core 7 appears therein, itwill be found that the self-induced voltage developed in winding 16a islarge and opposes the flow of any substantial current therethrough.Accordingly, no appreciable current can be caused to flow to load 2 fromthe secondary output winding 9 during such times. Ultimately, during thesame half cycle of voltage, the increase in source voltage results inincreased flux density almost exclusively in the control core 7, thiscore becoming saturated because ofthe-additional unidirectional fluxcirculated therein by the control winding 3. With this saturation, theflux in core 7 is unchanging, such that winding ltla no longer acts as ahigh impedance opposing current flow from source 1. The impedance :ofwinding half lda'then possesses a relatively low value, whereby the fullsupply voltage impressed across it ccasions a large current flow throughwinding half and :yields a high output'to the load 2, this effectcontinuing during the remainder of the half cycle under consideration.During the next half cycle of source voltage, which is of course ofopposite instantaneous polarity, the primary winding half 1012 conductscurrent and occasions delivery of output to the load in the same manner.Adjustment'of the amount of DC. control current flowing in the controlsignal winding 3, as by adjustment of thecontrol circuit impedance 4,alters the time during each half cycle of applied A.-C. when the controlcores '7 and 8 fire or saturate, and, hence, this adjustment results ingreater or lesser current being delivered to loadZ.

One important aspect of the aforesaid operation is best appreciated byassuming that one of the two control cores 7 and 8 is absent from thecircuit of Figure -1. If core 8:is omitted,.for example, and if only theusual low impedancesare presentin the control circuit, then controlwinding 3 functions as a low-resistance or shorted'winding about core 7and would draw relatively largepower'from the source, in the manner of atransformer secondary, before the firing orsaturating times in core 7.Correspondingly large load currents would then be drawn throughthesecondary load winding 9 during such intervals, because of thelargefluxes traversing the controlled core 6. Net effects are loss of gainand undesired dissipation of power inthe control circuit. While highimpedances in the control circuit might offset these particular defectssomewhat, then matching of the controlcircuit elements would bedifiicult and the required high impedances would be of undesired sizeand weight. With the other control core, 8,'in position, however, andwithcore 8 remaining essentially passive during firing cycles of core 7because of the blocking actions of rectifier 12, it is found thatcontrol winding 3 cannot act as a shorted winding and cause thementioned difiiculties. This is explained by the fact that the addedcontrol core 3 effectively increases the inductive impedance of winding3 to a high value during the critical intervals. Thus, no additionalimpedance is required in the control circuit and only the usual lowimpedance control circuit elements are required. Gain and losscharacteristics remain outstanding. Core '7 of course interchangesfunctions with core 8 when the primary winding 1% about the latter isexcited.

A further important feature is foundin the operation whereby the closedcontrol cores each convey magnetic flux periodically from theirrespective primary windings in accordance with unidirectional excitingcurrent. This results from the blocking actions .of .the rectifiers illand 12. It has been found that cores which carry fluxes in accordancewith an alternating current, as distinguished from pulsatingunidirectional current, tend to become demaguetized such that greatercontrol M. M. F. is needed to produce saturations. Accordingly, thefluxes (associated with the unidirectional exciting currents) in controlcores 7 and 8 by primary halves 16a and 1% enable lesser controlampere-turns to regulate the core saturations, and the system gain ismost advantageously heightened.

One configuration which may be assumed by cores and windings related inthe manner of those schematically portrayed in Figure l is presented inFigure 2, the components which are full counterparts of those in Figure1 being identified by the same reference characters with prime accents.The rectangular windowed cores 6', 7, and 8 are of the usual laminatedconstruction, and are each preferably fabricated from two U-shapedhalves which may be fitted about .the pre-wound cooperating windings.The two control cores 7' and .8 are disposed one each on outermost sidesof parallel legs of the controlled core 6, and may be somewhat smallerthan the controlled core. In this construction the control signalwinding arrangement .is divided into two equal serially coupled halves3a and 3b, each of which encircles a different one of the control cores6' and 7 this division.

wise conveniently divided into two serially-coupled aiding halves, 9aand 919, on thecontrolled cores 6'.

Direct current output to a load may be provided without furtherrectification through circuitry such as that of Figure 3 or Figure 4.The bi-phase arrangement in Figure 3 cmploysa pair ofrectifiers, .13 and14, which feed a D.-C. load 15 coupled between their junction and acenter tap 16 between secondary winding halves 17 and 18. It should benoted, in this instance, that these secondary winding halves eachencircle the controlled core 19 and a different one of the control cores2%"; and 21, and that the primary winding 22 excited by A.-C. source 23surrounds only the controlled core The-primary and secondary windingsare thus interchanged in relation tothe orientations .ofcorrespondingwindings of the circuit in.-Figure 1. Control signal winding 24encircles both of the control cores .20 and 21,

or, as in the case of structure ofFigure 2, maytakethe form of twoserially-coupled halves reach surrounding a different one of thesecontrolzcores.

The A.-C. signals developed by primary winding 22 are insufiicient tocause saturation of the controlled core 19, and during the first portion.of each half cycle of primary excitation, no appreciable signals aregenerated in the secondary winding halves 17 and 18. 'This is truebecause the control cores 20 and 21 are unsaturated during suchintervals, whereby both of the secondary halves 17 and 18 act as verylarge inductive i-mpedances. If it be considered that thevinstantaneoussignals appearing in'secondary winding half 17 are increasing from azero level'in'a sense to cause current flow through itand rectifier 13and load 15, theaforesaid inductive impedance effects willprecludesuchcurrent flow until, ultimately, the'flux density in control core 2%causes saturation thereof. This saturation results in part from thefluxes occasioned by winding 17 and in part from the unidirectionalfluxcirculated by control winding .24. Upon occurrence .-of this saturation,the effective inductance of the surrounding winding half 17 is minute,and large currents immediately flow to the load and continue to flowuntil the polarity reverses. Rectifier 14 blocks current during thisparticular half cycle but conducts duringthe otheralternate half cycleswhen its associated secondary windinglhalf .18 functions in theaforesaid manner. The load .15 thus receives periodic unidirectionalcurrent pulses. Direct current from source 25, as regulatedrbythevariable low ;im-

27, 28, 29, and 30 which accomplish this known form of output circuitinterconnection, the remaining components of the system there portrayedbeing identified by the same reference characters appearing with likecomponents in Figure 3, with distinguishing prime accents added.Operation otherwise corresponds to that of the arrangement of Figure 3.

' Multiple phase systems may employ these teachings as well. In Figure5, for example, controlled direct current is supplied to the load 31responsive to three-phase excitation applied across supply terminals 32and further responsive to the control exercised by the usual D.-C.control circuit coupled across the control terminals 33. It will berecognized that each of the transformer-amplistat units 34, 342, and 343is of the center-tap type described in connection with the illustrationin Figure 3. The unit for one phase includes controlled core 35 andcontrol cores 36 and 37, the primary winding 38 being mounted about thecontrolled core 35, the secondary winding halves 39 and 40 eachencircling the controlled core and a different one of the control cores36 and 37, and the two serially-coupled halves 41 and 42 of the controlsignal winding each encircling a different one of the control cores 36and 37. A pair of rectifiers, 43 and 44, completes the circuitry of thistransformer-amplistat unit, a different one being coupled seriallybetween the load 31 and each of the ends of secondary halves 39 and 40.The other two units, 342 and 343, are of like construction, andcorresponding components are thus identified by reference charactershaving the same first two digits. Each unit operates in the mannerearlier set forth with reference to the unit of Figure 3, except thateach is separately energized by A.-C. signals of a different phase fromthe three-phase supply terminals 32.

The three-phase transformer-amplistat schematically depicted in Figure 6likewise delivers D.-C. output to a suitable load 45 responsivetothree-phase excitation from across terminals 46 and responsive tocontrol exercised by the customary control circuit coupled acrosscontrol terminals 47. Connections are those for three-phase full wavebridge operation, however, with either full or partial control dependingupon certain couplings described later herein. Considering thearrangement of but one of the three cooperating units, 48, it is foundthat it comprises a controlled core 49, a pair of control cores 5t] and51, an A.-C. primary winding 52 about the controlled core, a pair ofoppositely-wound secondaries 5'3 and 54 each encircling the controlledcore and a different one of the two control cores 50 and 51, and a pairof serially-coupled primary winding halves 55 and 56 each surrounding adifferent one of the two control cores. Units 482 and 483 are of likeconstruction, with corresponding components identified by referencecharacters having the same first two digits. The three sets of D.-C.control signal windings are serially connected in the D.-C. controlcircuit through terminals 47, and each of the primaries 52, 522, and 523is connected across a different one of the three phases of excitationappearing at supply terminals 46.

Center tap connections between the oppositely-wound secondary halves ofeach unit are brought out to terminals 57, where they may be shorted orwhere additional threephase excitation may be applied. Ends of theseries-connected secondary halves are coupled across the load 45 throughoppositely-polarized rectifiers, such as rectifiers 58 and 59 in thecase of unit 48.

Operation with terminals 57 shorted is comparable to a three-phasefull-wave bridge system, in that there is an ordered sequence of thetimes when the various control cores saturate and when the control corefluxes are reset, the sequence being determined by the phase rotation ofthe three-phase supply connected to terminals 46. Assuming, for example,that core 50 saturates, it will be found that previous to this eventeither core 512 or core 513 is saturated, which one being determined bythe phase rotation of the supply as connected to terminals 46.Therefore, assuming that when core 50 saturates core 512 is saturated,coils 53 and 542 are connected serially by either shorted terminals 57or by a three-phase supply connected to terminals 57. The inducedinstantaneous voltages in coils 53 and 542 and the instantaneous voltageof the polyphase supply as connected to coils 53 and 542 at terminals57, if the three-phase supply is used instead of a short circuit atterminals 57, are all additive and of a polarity to cause current toflow through rectifier 58, through the load 45, through rectifier 592,through coil 542, through either a short circuit at terminals 57 orthrough a branch of a polyphase power supply connected to terminals 57,and through coil 53. Next, core 513 will saturate, and a path consistingof rectifier 593, coil 543, and a different branch of the polyphasesupply connected to terminals 57, if used, will replace the pathconsisting of rectifier 592, coil 542, and the previously used branchconnected between coil 53 and coil 542 at terminals 57 in the previouslydescribed loop supplying current through the load. This switching ofpaths supplying current to the load will occur simultaneously with theevent of a control core flux being increased to saturation. Each of thesix control cores 5%, 51, 502, 512, 563, and 513 is normally saturatedonce per cycle of the supply frequency, such that the switching of pathssupplying current to the load will consist of six different events aspreviously described, and the sequence will be repetitive each cycle ofthe supply frequency. Load current control is exercised by the D.-C.control signals impressed by way of terminals 57, these signalsgoverning the saturation periods in the control cores.

In some applications, it is not essential that the fullest control beexercised by the control signal circuitry, and a saving in equipmentsize and weight may be effected by applying part of the three-phaseexcitation across the terminals 57. These signals are phased properly,such that they will be rectified and delivered to the load additivelywith the signals controlled by the transformeramplistat units in themanner last described.

One preferred construction of cores and windings useful in the practiceof three-phase transformer-amplistat circuitry appears pictorially inFigure 7. Although winding interconnections and couplings to othercircuit com ponents are not illustrated, it should be apparent thatthese may be made in accordance with the teachings set forth elsewhereherein. The controlled core 66 in this arrangement is a unitary elementwhich, it will be shown, serves the functions of three separate closedcontrolled cores. This core is of a spiral-wound construction whereinthe two spiral-wound inner portions 61 and 62 are placed in an abuttingrelationship to formv a center leg 60a of a given thickness and whereinan outer portion 63 encompasses these inner portions to build up thethickness of the two outer legs 69b and 660 to substantially thethickness of center leg 60a. Following the core winding operations, theunitary core 60 is preferably cut into two E-shaped halves which may befitted together through the associated electrical windings nextdescribed. Three electrical primary windings 64, 65, and 66 surround thecore legs 60b, 60a, and 690, respectively, over substantially theirentire lengths. To secondary winding halves in turn encircle each of theprimaries, and

each secondary simultaneously encircles a different closed control core,which is ,of relatively small size. For example, secondaries 67 and 68surround primary 64 and controlled core leg 6%, and at the same time theturns of these secondaries pass through the windows of closed controlcores 6% and ilt, respectively. These two control cores are convenientlydisposed one on each side of the controlled core leg 6%. Control signalwindings "/1 and '72 also encircle the control cores 6% and 7%respectively. Similarly, secondaries '73 and "74 an core legs 6% andthread separate control cores, only one of which, core "F5, is visiblein the drawing together with its associated control signal winding 76.Two secondaries 7'7 and 78 also appear about core leg 58c and likewisethread separate control cores, the control co e 79 and its controlsignal winding '89 alone being visible in the View taken in Figure 7.

While particular embodiments of this invention have been shown anddescribed, it will occur to those sl-iillcd in the art that variouschanges and modifications can be made without departure from theinvention, and therefore, it is aimed in the appended claims to coverall such changes and modifications as fall within the true spirit andscope of the invention.

What l claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Apparatus for effecting controlled electrical excita tion of a loadcomprising a source of periodically varying voltage, a controlledmagnetizable core member and at least one pair of saturable control coremembers each defining a magnetic flux path, a source of control current,first winding means energized by said control current source anddisposed to vary the saturation characteristics of both of said controlcore members simultaneously responsive to said control current, secondwinding means linking said controlled core member, rectifier means,third winding means having two portions each linking both saidcontrolled core member and a dilferent one of said control core membersand each coupled with said rectifier means to conduct current but onedirection, means coupling said source of periodically varying voltagewith one of said second and third winding means, and means coupling theother of said second and thir" winding means with said load.

2. Apparatus for effecting controlled electrical excitation of a loadcomprising a source of periodically varying voltage, a controlledmagnetizable core and a pair of saturable control cores each defining amagnetic fiux path, a source of adjustable control current, firstwinding means energized by said control current source and linking bothof said control cores to vary the saturation characteristics of both ofsaid control cores simultane ously responsive to adjustments of saidcontrol current, second windin meansencircling only said controlledcore, third winding means having two winding portions each encir -ingboth said controlled core and a different one of said control cores,means coupling said source of periodically varying voltage with one ofsaid second and third winding means, means coupling said load with theother of said second and third winding means, and rectifier meanscoupled in circuit with said winding portions of said third windingmeans to conduct current in a different direction through each of saidwinding portions.

3. Apparatus for eliecting controlled electrical excitation of a loadcomprising a source of periodically varying voltage, a controlledmagnetizable core member and at least one pair or" saturable controlcore members each defining a closed magnetic flux path, a source ofadjustable unidirectional control current, first winding means energizedby said control current source and linking said control core members tovary flux density in each of said control core members simultaneouslybelow the levels of saturation in said control core members, secondwinding means encircling said controlled core member, third windingmeans having two portions each encircling both ,said

controlled core member and a different one of said pair of control coremembers, said second and third windings being brought into the relationof cooperating transformer windings by encirclement of said controlledcore member, means coupling said source of periodically varying voltagewith one of said second and third winding means, said controlled coremember and said one of said windings being proportioned such that saidcontrolled core member is not saturated responsive to said periodicallyvarying voltage, means coupling said loadwith the other of said secondand third winding means, and rectifier means coupled in circuit withsaid winding portions of said third winding means to conduct current ina different direction through each of said winding portions whereby eachof said pair of control core members is saturated independently byunidirectional fiuxes and said first winding means possesses highimpedance opposing induced -A.-C. signals therein.

4. Apparatus for efiecting controlled A.-C. electrical excitation of aload comprising a source of alternating voltage, a controlledmaguetizable core member and at least one pair of saturable control coremembers each defining a magnetic flux path, a source of adjustableunidirectional control current, control winding means energized by saidcontrol current source and linking both of said control core members,primary winding means having two winding portions each encircling bothsaid controlled core member and a different one of said control coremembers, secondary winding means encircling only said controlled .coremember and coupled with said load, means coupling said winding portionsof said primary winding means with said source of alternating voltagefor excitation thereby, and rectifier means in circuit with said primarywinding means connected to conduct current from said alternating voltagesource to each of said primary winding portions only during differentalternate half cycles of said alternating voltage.

5. Apparatus for effecting controlled electrical excitation of a load,comprising a source of alternating voltage, a controlled magnetizablecore member and at least one pair of saturable control core memberseachdefining a magnetic flux path, a source of adjustable unidirectionalcontrol current, control winding means energized by said control currentsource and linking both or" said control core members, primary windingmeans encircling only said controlled core member and coupled with saidsource of alternating voltage for excitation thereby, secondary windingmeans having two winding portions each encircling both said controlledcore member and a different one of said control core members, meanscoupling said winding portions of said secondary winding means ith saidload, and rectifier means in circuit with said secondary winding meansconnected to conduct current from each of said winding portions to saidload only during intervals when signals of a predetermined polarity areinduced therein.

6. Apparatus for effecting controlled electrical excitation of a loadcomprising a source of alternating voltage, a controlled magnetizablecore member and at least one pair of saturable control core members eachdefining a magnetic flux path, a source or" adjustable unidirectionalcontrol current, control winding means energized by said control currentsource and linking both of said control core members to vary the fluxdensity in each of said control core members simultaneously below thelevels of saturation in said control core members, primary winding meansencircling only said controlled core member and coupled with said sourceof alternating voltage for excitation thereby, said controlled coremember and said primary winding means being proportioned such that saidcontrolled core member is not saturated responsive to said alternatingvoltage, secondary winding means having two Windingportio-ns eachencircling both said controlled core member and a difierent one of saidcontrol core members, said primary and secondary winding-means beingbrought into the relation of cooperating primary and secondarytransformer windings by encirclement of said controlled core member,means coupling said load with said secondary winding means, andrectifier means coupled in circuit with said winding portions of saidsecondary winding means to conduct current through each of said windingportions to said load only during intervals when signals ofpredetermined polarity are induced therein whereby each of said pair ofcontrol core members is saturated independently by unidirectional fluxesand said control winding means possesses high impedance opposing inducedA.-C. signals therein.

7. Apparatus for effecting controlled electrical excitation of a loadcomprising a source of three-phase alternating voltage, a source ofadjustable unidirectional control current, three magnetic units eachcomprising a controlled core member and a pair of control core members,control winding means linking all of said control core members of saidthree units and excited by said source of control current to vary theflux density in each or" said control core members simultaneously belowthe levels of saturation of said core members, three primary windingmeans each encircling a different one of said controlled core members ofsaid units and each coupled with said three-phase source for excitationby voltage of a different phase, three secondary winding means eachhaving two winding portions each encircling said controlled core memberand a different one of said control core members of a different one ofsaid units, means coupling said secondary Winding means with said load,and rectifier means in circuit with said load and with each of saidWinding portions of said secondary winding means conducting current onlyin a predetermined direction through said winding portions.

8. Apparatus for effecting controlled electrical excitation of a loadcomprising a source of three-phase alternating voltage, a source ofadjustable unidirectional control current, a three-legged closedmagnetic core, three primary windings each encircling a different one ofsaid core legs and each coupled with said three-phase source forexcitation by voltage of a difierent phase source for excitation byvoltage of a different phase, three pairs of closed magnetic controlcores disposed one pair each alongside a different one of said corelegs, three secondary winding means each having two winding portionsboth encircling a difierent one of said core legs and each alsoencircling a different one of said control cores alongside said one ofsaid core legs, control windings each encircling a different one of saidcontrol cores and coupled in series across said source of adjustableunidirectional control current, means coupling said secondary windingmeans with said load, and rectifier means in circuit with said load andwith each of said winding portions of said secondary winding meansconducting current only in a predetermined direction through saidwinding portions.

9. Apparatus for effecting controlled electrical excitation of a loadcomprising a source of periodically varying voltage, a controlledmagnetizable core member and at least one pair of saturable control coremembers each defining a magnetic flux path, a source of adjustableunidirectional control current, first winding means energized by saidcontrol current source and linking both of said control core members tovary the flux density in said control core members below the levels ofsaturation thereof, second winding means encircling said controlled coremember, third winding means having turns encircling said controlled coremember and one of said control core members, said second and thirdwinding means being brought into the relation of cooperating transformerwindings by encirclement of said controlled core member, means couplingsaid source of periodically varying voltage with one of said second andthird winding means, said controlled core member and said one of saidwinding means being proportioned such that said controlled core memberis not saturated responsive to said periodically varying voltage, meanscoupling said load with the other of said second and third windingmeans, and rectifier means coupled in circuit with said third windingmeans to conduct current in but one direction through said turns wherebysaid one of said pair of control core members is saturated byunidirectional fluxes and said first winding means possesses highimpedance opposing induced A.-C. signals therein.

References Cited in the file of this patent UNITED STATES PATENTS2,216,631 Sorensen Oct. 1, 1940 2,745,056 Zucchino May 8, 1956 2,762,969Fingerett Sept. 11, 1956

